For many years coagulation instruments have been provided to measure the clotting time of human plasma through the use of photometric techniques. In such instruments, a light beam is directed through a plasma sample to which a suitable reagent has been added. The light passing through the sample is sensed by a light sensitive element or photocell, and as the plasma goes through the clotting process and becomes less translucent, the light falling on the photocell decreases. Thus, an electrical signal from the photocell can be processed by electronic circuitry to provide an accepted measure of clotting time. In instruments that are available in the marketplace, the electronic circuitry generally includes means that provide a first or a second derivative signal of the optical density signal obtained at the photocell. At a point determinative of clot formation, the derivative signal will stop a timer that had been started when the reagent was added to the plasma sample to initiate the clotting process. The timer output is taken as a measure of the clotting time of the plasma. Normal clotting time for prothrombin time measurements is about 12 seconds while for activated partial thromboplastin time measurements a normal clotting time will be between 20 and 35 seconds.
A modern coagulation instrument is a complex device comprising many subsystems that enable automation of plasma coagulation tests such as those referred to above. For example, there are cooling elements that maintain plasma samples and reagents at desired storage temperatures, i.e., 8.degree. C., and heating elements that raise the temperatures of the plasma and the reagents to 37.degree. C. when they are mixed to initiate clot formation. Of course, there is the electronic circuitry that responds to the photocell signal to detect the formation of a clot.
When erroneous results are obtained from the instrument, as indicated by clotting times that materially deviate from the expected clotting times obtained on standardized control plasma, it becomes a significant problem for the technician using the instrument to determine which element or component of the instrument is faulty or malfunctioning, if indeed the failure is due to the instrument itself. Erroneous results can also result, for example, from the use of substandard reagents that may have deteriorated during storage, but it is the initial tendency to blame the instrument for the error. Certain operational characteristics of the instrument, such as the temperature condition of a component or reagent volumes delivered by pumping mechanisms, can be checked by conventional means, but there is no convenient way for the technician to check that the electronic circuitry is performing as it should.